Steps toward multiple dbs available -- working

There will need to be some more refactoring for these changes to be
considered complete/correct, but for now it's working.

new daemon cli argument "--db-type", works for LMDB and BerkeleyDB.

A good deal of refactoring is also present in this commit, namely
Blockchain no longer instantiates BlockchainDB, but rather is passed a
pointer to an already-instantiated BlockchainDB on init().

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -733,13 +733,6 @@ void BlockchainLMDB::open(const std::string& filename, const int mdb_flags)
   // from here, init should be finished
 }
 
-// unused for now, create will happen on open if doesn't exist
-void BlockchainLMDB::create(const std::string& filename)
-{
-  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
-  throw DB_CREATE_FAILURE("create() is not implemented for this BlockchainDB, open() will create files if needed.");
-}
-
 void BlockchainLMDB::close()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);

src/blockchain_db/lmdb/db_lmdb.h

@@ -24,6 +24,7 @@
 // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
 
 #include "blockchain_db/blockchain_db.h"
 #include "cryptonote_protocol/blobdatatype.h" // for type blobdata
@@ -116,8 +117,6 @@ public:
 
   virtual void open(const std::string& filename, const int mdb_flags=0);
 
-  virtual void create(const std::string& filename);
-
   virtual void close();
 
   virtual void sync();
@@ -302,7 +301,6 @@ private:
 
   MDB_dbi m_spent_keys;
 
-  bool m_open;
   uint64_t m_height;
   uint64_t m_num_outputs;
   std::string m_folder;